One skill that should be in every home brewers arsenal is understanding how and why to make a yeast starter. Yeast starters provide a healthy, metabolically active pitch of yeast that will work to give you a clean fermentation with a cell count appropriate to your starting gravity. Starters can be easily made using just a simple growler with a stopper or airlock, or using more advanced techniques and equipment such as an Erlenmeyer flask and a stir plate.

The first step in making a starter is practicing proper sanitation. Starters can be very susceptible to contamination from wild yeast and bacteria so it is very important to practice good and thorough sanitation techniques. Anything that is going to come in contact with your starter needs to be cleaned and soaked in a strong sanitizer solution. I prefer either Star San or Sani Clean mixed to their prescribed strengths Allow all of your vessels and utensils to soak for 10 minutes. This is definitely overkill but I would rather err on the side of caution.

Since starters are essentially small batches of beer, the process is very simple. The target gravity of our starter wort is 1.040. This is suitable for a fresh, well handled smack pack or vial of liquid yeast. If the yeast is not fresh, or if you suspect that it has been handled poorly, your target gravity for your starter wort should be around 1.020 An OG of 1.020 is also recommended if you are trying to grow yeast from a bottle conditioned beer.

To achieve our starting gravity, and for simplicity sake, it is easier to measure our water and DME in metric units using a 10 to 1 ration of water to DME. For example a two liter starter would require two liters of water and 200 grams of DME. This will get you very near 1.040 every time. Add 1/8 teaspoon of yeast nutrient and boil the resulting wort for 10 minutes to sanitize, then chill to pitching temperature and transfer to a clean and sanitized growler fitted with a stopper and airlock. Aerate the wort by either vigorous shaking, an aquarium pump with HEPA filter and aeration stone, or pure oxygen with an oxygenation stone. To reduce the risk of contamination, sanitize the outside of your vial or smack pack before opening. Add your yeast and swirl your growler to mix evenly. If possible, shake the growler every hour, or as often as possible to keep the yeast in suspension to help promote maximum growth.

If you are using an Erlenmeyer flask and stir plate, your job is much easier. If your flask is made of borosilicate glass you can mix your water and DME in the flask and bring to a boil directly on your stove. No need to transfer from your pot to your flask. The boil can be done in the flask itself. Once the boil is complete, cover the top of the flask with a piece of sanitized tin foil or plug with a sanitized foam plug. To chill, place the flask in a hot water bath and slowly add ice to chill. While some people will say that the borosilicate glass can handle the shock of going from boil directly to ice bath, one shattered flask was enough to convince me that it was not a good idea. Once chilled to pitching temperature, add your yeast and place on your stir plate. The action of the stir plate will draw in oxygen and expell carbon dioxide eliminating the need to aerate the wort as you would using a growler and airlock

With either method, maximum growth will be achieved within 18 to 36 hours and will provide an adequate number of yeast to ferment a beer of up to a 1.075 starting gravity. Beers with an OG of greater than 1.075 will require additional steps to increase yeast population to handle the additional sugars.